The term nitrile rubber, also referred to as “NBR” for short, refers to rubbers which are copolymers or terpolymers of at least one α,β-unsaturated nitrile, at least one conjugated diene and, if desired, further copolymerizable monomers.
Hydrogenated nitrile rubber, also referred to as “HNBR” for short, is produced by hydrogenation of nitrile rubber. Accordingly, the C═C double bonds of the copolymerized diene units are entirely or partly hydrogenated in HNBR. The degree of hydrogenation of the copolymerized diene units is usually in the range from 50 to 100%.
Hydrogenated nitrile rubber is a specialty rubber which has very good heat resistance, excellent resistance to ozone and chemicals and excellent oil resistance.
The abovementioned physical and chemical properties of HNBR are combined with very good mechanical properties, in particular a high abrasion resistance. For this reason, HNBR has found widespread use in a variety of applications. HNBR is used, for example, for seals, hoses, belts and damping elements in the automobile sector, also for stators, oil well seals and valve seals in the field of oil production and also for numerous parts in the aircraft industry, the electronics industry, mechanical engineering and shipbuilding.
HNBR grades which are commercially available on the market usually have a Mooney viscosity (ML 1+4 at 100° C.) in the range from 55 to 105, which corresponds to a weight average molecular weight Mw (method of determination: gel permeation chromatography (GPC) against polystyrene standards) in the range from about 200 000 to 500 000. The polydispersity indices PDI (PDI=Mw/Mn, where Mw is the weight average molecular weight and Mn is the number average molecular weight) measured here give information about the width of the molecular weight distribution and frequently have a value of 3 or above. The residual double bond content is usually in the range from 1 to 18% (determined by IR spectroscopy).
The processability of HNBR is greatly restricted by the relatively high Mooney viscosity. For many applications, it would be desirable to have an HNBR grade which has a lower molecular weight and thus a lower Mooney viscosity. This would greatly improve the processability.
Numerous attempts have been made in the past to shorten the chain length of HNBR by degradation. For example, it is possible to achieve a decrease in the molecular weight by thermomechanical treatment (mastication), e.g. on a roll mill or in a screw apparatus (EP-A-0 419 952). However, this thermomechanical degradation has the disadvantage that functional groups such as hydroxyl, keto, carboxylic acid and ester groups are incorporated into the molecule by partial oxidation and, in addition, the microstructure of the polymer is substantially altered.
The preparation of HNBR having low molar masses corresponding to a Mooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a number average molecular weight of about Mn<200 000 g/mol was not possible by means of established production methods for a long time, since, firstly, a step increase in the Mooney viscosity occurs on hydrogenation of NBR and, secondly, the molar mass of the NBR feedstock to be used for the hydrogenation cannot be reduced at will, since otherwise work-up in the available industrial plants is no longer possible because of excessive stickiness. The lowest Mooney viscosity of an NBR feedstock which can be worked up without difficulties in an established industrial plant is about 30 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity of the hydrogenated nitrile rubber obtained using such an NBR feedstock is in the order of 55 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity is in each case determined in accordance with ASTM standard D1646.
In the more recent prior art, this problem is solved by reducing the molecular weight of the nitrile rubber before hydrogenation by degradation to a Mooney viscosity (ML 1+4 at 100° C.) of less than 30 Mooney units or a number-average molecular weight of Mn<70 000 g/mol. The decrease in the molecular weight is achieved here by means of a metathesis reaction in which low molecular weight 1-olefins are usually added. The metathesis reaction is advantageously carried out in the same solvent as the subsequent hydrogenation reaction, so that the degraded NBR feedstock does not have to be isolated from the solvent after the degradation reaction before it is subjected to the hydrogenation. To catalyse the metathetic degradation reaction, use is made of metathesis catalysts which are tolerant of polar groups, in particular nitrile groups
WO-A-02/100905 and WO-A-02/100941 describe a process which comprises the degradation of nitrile rubber starting polymers by olefin metathesis and subsequent hydrogenation. Here, a nitrile rubber is reacted in a first step in the presence of a coolefin and a specific complex catalyst based on osmium, ruthenium, molybdenum or tungsten and the product is hydrogenated in a second step. In this way, it is possible to obtain hydrogenated nitrile rubbers having a weight average molecular weight (Mw) in the range from 30 000 to 250 000, a Mooney viscosity (ML 1+4 at 100° C.) in the range from 3 to 50 and a polydispersity index PDI of less than 2.5.
Metathesis catalysts are described in general terms in, for example, WO-A-96/04289 and WO-A-97/06185. The have the following in-principle structure:
where M is osmium or ruthenium, R and R1 are organic radicals having a large structural variety, X and X1 are anionic ligands and L and L1 are uncharged electron donors. In the literature, the customary term “anionic ligands” always refers, in the context of such metathesis catalysts, to ligands which, when they are regarded separately from the metal centre, would be negatively charged for a closed electron shell.
Such catalysts are described as suitable for ring-closing metatheses (RCM), cross-metatheses (CM) and ring-opening metatheses (ROMP).
The metathesis of nitrile rubber can be carried out successfully using catalysts from the group of “Grubbs (1) catalysts”. An example of a suitable catalyst is the bis(tricyclohexylphosphine)benzylidene ruthenium dichloride catalyst shown below.

After metathesis and hydrogenation, the nitrile rubbers have a lower molecular weight and also a narrower molecular weight distribution than the hydrogenated nitrile rubbers which can be produced according to the prior art.
However, the amounts of Grubbs (I) catalyst employed for carrying out the metathesis are large. According to the examples in WO-A-03/002613, they are 307 ppm and 61 ppm of Ru. The reaction times required are also long and the molecular weights even after degradation are still relatively high (see Example 3 of WO-A-03/002613, where Mw=180 000 g/mol and Mn=71 000 g/mol).
US 2004/0127647 A1 describes blends based on low molecular weight HNBR rubbers having a bimodal or multimodal molecular weight distribution and also vulcanizates of these rubbers. To carry out the metathesis, 0.5 phr of Grubbs I catalyst, corresponding to 614 ppm of ruthenium, is used according to the examples.
Furthermore, WO-A-00/71554 discloses a group of catalysts which are described as “Grubbs (II) catalysts” in the art. If such a “Grubbs (U) catalyst”, e.g. 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidenylidene)(tricyclohexylphosphine)ruthenium(phenylmethylene)dichloride, is used for the NBR metathesis, this occurs successfully even without use of a coolefin (US-A-2004/0132891). After the subsequent hydrogenation, which is carried out in situ, the hydrogenated nitrile rubber has lower molecular weights and a narrower molecular weight distribution (PDI) than when catalysts of the Grubbs (I) type are used. In terms of the molecular weight and the molecular weight distribution, the metathetic degradation using catalysts of the Grubbs II type proceeds more efficiently than that using catalysts of the Grubbs I type. However, the amounts of ruthenium necessary for this efficient metathetic degradation are still relatively high and long reaction times are still required.

In all the abovementioned processes for the metathetic degradation of nitrile rubber, relatively large amounts of catalyst have to be used and long reaction times are required to produce the desired low molecular weight nitrile rubbers.
In J. Am. Chem. Soc. 1997, 119, 3887-3897, it is stated that in the following ring-closing metathesis of diethyl diallylmalonate
the activity of the catalysts of the Grubbs I type can be increased by additions of CuCl and CuCl2. This increase in activity is explained by a shift in the dissociation equilibrium as a result of phosphane ligand dissociated from the Grubbs I catalyst being scavenged by the copper ions to form copper-phosphane complexes. However, it is stated in J. Am. Chem. Soc. 1997, 119, 3887-3897, that the addition of phosphanes in an amount of from 0.25 to 1.0 equivalent per 1 equivalent of the ruthenium-carbene complex results in a reduction in the reaction rate of the ring-closing metathesis.
It is therefore an object of the invention to find catalyst systems which have an increased activity when used in the metathetic degradation of nitrile rubbers in order to reduce the amounts of catalysts necessary, in particular the amounts of noble metal present, and decrease the reaction rate.